Scanning Acoustic Microscopy (SAM) is a non-destructive method used to detect cracks, voids, and de-laminations occurring in the surface/sub-surface of different kinds of objects under test. One application of Scanning Acoustic Microscopy is defect detection in semi-conductor packages. Examples of semi-conductor packages include wire bond/ribbon bond packages, flip chip packages, tape automated bond packages, ball grid array packages, chip scale packages, printed circuit boards and bonded wafers (including blank, patterned, MEMS, and 3D Interconnect wafers). Apart from defect detection, SAM is also helpful in metrology applications such as measurement of material (bond layer/wafer) thickness, trench depth measurement and evaluation of wafer pair alignment post bonding.
In a pulse echo mode of imaging, an ultrasonic transducer generates an ultrasonic pulse (coupled through water) and, as the ultrasound travels through the object under test, a portion of the signal is reflected. The reflection depends on changes in acoustic impedance in the object. The reflected signal is converted back into an electrical signal and, in turn, into a pixel value for the location being tested. The conversion uses a signal path typically including a transducer, a pulser/receiver, and an analog-to-digital (A-D) conversion board. Once the pixel value is obtained for a single location with Cartesian coordinates (x, y), the motion controller moves the transducer to the next pixel coordinate, based on the scan resolution. Thus, a raster scan is performed on the entire region of interest to obtain a SAM image of the sample.
The throughput of the SAM system is limited by the speed of the motors that move the transducer across the surface of the object under test (the Scan-X and the Scan-Y axes, for example) and the repetition rate of the pulser/receiver. In order to minimize the duration of a scan, it is desirable to increase the throughput of SAM systems. SAM systems may be used both in the front end (wafer) of a production line and at the back end (package) of the line.
SAM systems are available in both manual and automated modes. Manual systems require the user to manually load and unload the samples (packages/wafers) before and after the scan. In an automated wafer inspection system, the cassettes or Front Opening Unified Pods (FOUP's) of wafers are placed in the loading stations and a robot transfers the wafers into a scanning chamber. Once scanning is completed, the robot removes the wafers from the scanning chamber, moves them to a drying chamber and then back into the appropriate wafer slot on the cassette/FOUP in the loading station. In automated package inspection systems, the Joint Electron Devices Engineering Council (JEDEC) standard trays populated with the samples are placed in the loading station. The samples are picked up and placed in the scanning station row by row. After scanning and drying is completed, the samples are picked up and placed back into the JEDEC trays. The tray is then transferred to an unloading station on a conveyor belt.
The throughput of the system is generally defined in terms of the time taken to inspect one object from the cassette/FOUP/tray (from inputting a dry object to outputting a dry object). The total time includes time for scanning, data acquisition, analysis, and handling. Optimizing any, or all, of these parameters will improve throughput. For example, designing the system to perform operations in parallel, rather than in series, minimizes the impact of the handling and analysis times on the overall throughput. Automated systems, where three wafers are in the system at the same time (i.e. one wafer at each of the pre-aligner, scan station, and dryer station), have greatly reduced the handling time per wafer. Analyzing one wafer while the next wafer is being scanned reduces the analysis impact on throughput to a negligible amount. However, the dominant factor affecting throughput is the scan time itself. Optimizing the data transfer time for each scan line of data does have an impact on the throughput since the scan can run continuously rather than having to wait briefly at the end of each scan line for data transfer. However, even this improvement does not meet the industry requirements for throughput.
Previous work has focused on three methods for improving the scan time: multiple scan stations (i.e. multiple sets of raster scanner, transducer, and sample holder), multiple sample holders (one raster scanner and transducer combination moves over multiple sample holders), or multiple transducers (one raster scanner moves multiple transducers over one sample holder).
The first option (multiple scan stations) allows for completely separate motion over different samples at the same time. However, it requires multiple complete scanning stations, which increase the complexity and footprint of the system. This is a major consideration on a production floor, where space is at a premium. Also, if one scan station in the system has a problem, the whole system has to go off-line to be serviced. This has a major impact on throughput.
The second option (multiple holders) allows for one scan motion over multiple samples, which requires less duplication of scan station components and could allow for greater scan speeds since there is more distance over which acceleration can be applied. However, this option also requires an increase in system footprint. Further, if one sample holder has a problem, the whole system has to go off-line to be serviced.
The third option (multiple transducers) requires some added complexity for the acquisition paths, but the system footprint would be the same as for a single transducer—single scan station system. Also, if a problem occurs on this type of system, only that one scan station is affected. Other scan stations (systems) will keep running. Also, the multiple transducer (multi-transducer) approach, using N transducers, can potentially reduce the scan time by a factor of N.
The main challenge in the multi-transducer approach is the inherent variation in the performance of one transducer compared to another. Though the transducers may be within the required specification, there may be slight focal length differences, or variation in the strength of the signal of interest, the waveform shape of the signal, or the phase of the signal, for example. These variations in transducers can result in variations in the scanned image. The multi-transducer approach requires an image to be pieced together from multiple transducer data and transducer variations can result in an image with a patchwork appearance. This can also negatively impact analysis results for the sample. Previous work in the multi-transducer approach has allowed for independent focusing of each transducer to account for the differences in focal length between transducers. However, no prior work has addressed the signal strength and shape/phase variations.
In a SAM system, the characteristic of the signal path is dependent, in part, on the response of the one or more transducers in the path. If a transducer in the signal path is replaced, because of failure or other reason, the characteristic of the signal path is changed. This makes it more difficult to compare scans made using different transducers or using different SAM systems. In addition, the time-gates of the system may need to be checked and possibly repositioned, since the phase response of the new transducer may be different.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimension of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.